1. Field of the Invention
The present invention generally relates to a mobile communication system employing a Time Division Multiple Access (TDMA) scheme, more particularly to a mobile communication system and its adaptive modulation method for individually assigning a modulation scheme to each time slot.
2. Description of the Related Art
In a digital mobile communication system employing a TDMA scheme, the whole system bandwidth is used, and a fixed time period (referred to as frame) is divided into a plurality of unit times (referred to as time slots). Each of the time slots is used as a communication channel.
Also CDMA/TDD (Code Division Multiple Access/Time Division Duplex) scheme has been proposed in which the time slots are respectively code division multiplexed.
Further, the modulation schemes that can be employed in a mobile communication system may include phase shift modulation schemes such as BPSK, QPSK, and 8PSK, and quadrature amplitude modulation schemes such as 16QAM and 64QAM. For illustration, signal points in QPSK and in 16QAM are shown in FIG. 1A and FIG. 1B respectively.
In conventional mobile communication systems, some systems may fix a modulation scheme to use, or others may use a plurality of modulation schemes adaptively. In a typical adaptive modulation method, on the basis of, for example, the line quality of a radio link, when the line quality is fine, a modulation scheme having more bits per symbol is used in order to increase the amount of information that can be transmitted at a time, while, when the line quality is not fine, a modulation scheme having less bits per symbol is used in order to reduce the influence of noise.
However, in the conventional adaptive modulation method, a new modulation scheme has to be transmitted to a communicating station each time the modulation scheme to be used is changed. To this end, the amount of data of control signals to be transmitted is increased.
A method intended to solve this problem includes a method disclosed in Japanese Laid-Open Patent Application No.2000-324081. According to the disclosed method a receiving station (a mobile station) determines autonomously which modulation scheme is used on received signals based on the same.
However, such an autonomous determination method takes a long time for the determination process and makes the size of the mobile station larger. Also, depending on the accuracy of the determination, it could not reliably obtain a modulation scheme to be used.
As described above, the conventional adaptive modulation method cannot decrease the amount of data of the control signals at the same time that it reliably transmits the modulation scheme to be used to the receiving station.